A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask, a reticle, an array of individually controllable elements, etc. can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer or a flat panel display substrate). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Highly integrated circuits require small circuit elements. Since the radiation pattern shapes the circuit elements, the smallest feature size depends on the resolution achieved in the lithography exposure step, or the resolution of the projection device used to project the radiation pattern onto the substrate. According to the Raleigh criterion, this resolution is proportional to the wavelength λ of the projected light and to an adjustment factor k1, and inversely proportional to the sine function of the marginal, or capture, angle θ of the projection optics, where:Resolution=k1*λ/sin(θ)
The resolution can be decreased, i.e., improved, in various ways. First, the wavelength λ of the projected light can be decreased. A shorter wavelength may require different types of photoresist and a number of changes in the projection device, such as using a different light source and light filters, and special lenses for the projection optics. Second, the resolution can be decreased by decreasing the adjustment factor k1. Decreasing k1 may also require the use of different types of photoresist and high precision tools. Third, the marginal angle θ can be increased by increasing the size of the projection optics. The effect of increasing the marginal angle θ can be limited by the sine function above. One way to reduce the wavelength λ of the projected light is through the use of immersion lithography.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. The effect of the liquid can also be regarded as allowing the numerical aperture (NA) of the system to be higher than 1 and also increasing the depth of focus. Other immersion liquids have been proposed, including water with solid particles (e.g., quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid can lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate (the substrate generally has a larger surface area than the final element of the projection system).
A gap between the liquid supply system and the substrate allows these elements to move with respect to each other. Because of this gap, there is a need to have high surface tension between the immersion liquid and at least a “showerhead” or hood portion (showerhead and hood are used interchangeably throughout) of the liquid supply system to keep the immersion liquid from flowing through or breaking a meniscus formed at an edge of the gap. For example, a showerhead can be a portion of the liquid supply system that comprises inlet and outlet ports and/or channels. A problem that can arise in immersion lithography systems is formation of small contact angles between an immersion liquid and a surface of the substrate and the liquid supply system. The contact angle is defined by the surface energies between fluid and surface. Small contact angles mean large capillary forces, which may cause fluid break-through.
One concern in immersion lithography relates to ensuring purity and lack of contamination of the immersion liquid. In one example, the immersion liquid is recirculated using an injection system to inject the liquid into the volume between the projection optics and the substrate and an extraction or suction system to extract the liquid from the exposure area back into recirculation. However, the liquid can get contaminated, for example through receiving particles from the air or due to receiving material from the photoresist that is being exposed. Normally, filtering systems are in place to remove the contaminants.
Another concern, in a recirculation example, is that not all of the liquid that is injected into the exposure area may actually be recirculated due to the surface tension that exists between the liquid and a substrate surface. Although most of the liquid can be extracted, using the suction pressure of the extraction/recirculation system, some droplets of liquid remain on the surface of the substrate, together with their contaminants. Increasing the suction pressure generally does not help past a certain point because, although this will increase the recirculation speed, increased suction pressure may not address the problems caused by the surface tension of the liquid.
A further concern in immersion lithography is that above a certain scan speed the substrate will pull a film (or droplets) from the meniscus in a direction of the scan, i.e., cause a meniscus break. The speed at which the break occurs can be increased, allowing for faster scanning, by reducing a height of the meniscus (e.g., reducing a gap), which can have several effects. First, the moment of first water loss (e.g., meniscus break point) is shifted to a higher scan speed. Secondly, the amount of water that is lost is reduced, where height of water film˜height of meniscus*velocity of scan ^(⅔). Whenever water is lost from the meniscus, an “air knife” can be used to hold the lost water. Although reducing the gap is typically satisfactory, other problems can occur.
The amount of water that passes through the air knife depends on a pressure gradient of the air on the substrate surface. A reduced gap size can lead to a higher pressure gradient, which can lead to less water loss. So, on the positive side, smaller gaps between the hood and substrate lead to less water loss. However, there are also several negative effects of a smaller gap. First, the air velocity above the substrate surface can increase, which can enhance the evaporation of water from the surface and can lead to increased cooling of the substrate, which is undesirable. Secondly, the air knife may hold “outside” water on the outside of the air knife, but it is desirable that this water “flows back” (e.g., is recollected) into the meniscus when scanning back (at the advancing meniscus).
Accordingly, what is needed is an immersion lithography system and method that ensures that substantially all of immersion liquid is collected by an extraction system and/or that an optimally sized gap is formed to reduce liquid loss and increase recollection of a liquid at the gap.